There are many teachings and commercial applications relating to the production of consolidated composite wood products where a layup of discrete wood elements coated with an appropriate adhesive is consolidated by application of heat and pressure to bind the discrete wood elements together and thereby form a consolidated composite wood product. Sufficient heat is applied to heat the adhesive to a temperature sufficient to cure the adhesive (thermo-setting resin such as phenol formaldehyde, and the like).
U.S. Pat. No. 3,880,975 issued Apr. 29, 1975 to Lundmark describes the process of making a resin bonded hardboard by uniformly across the width of the layup, heating and compressing the layup to reduce its thickness slightly by a pair of heated nip forming rolls, then finally compressing and setting the resin in a hot plate press. This system is for the production of hardboard which generally employs very small wood particles and the prepressing stage is simply a minor precompression and heating stage wherein the pressing rollers are heated to transfer some heat at least to the surface of the layup being processed.
U.S. Pat. No. 4,216,179 issued Aug. 5, 1980 to Lamberts et al. discloses a system for making particleboard from wood particles in the form of chips or fibers or the like and wherein a high frequency preheating device directs high frequency energy into the layup uniformly across its width to preheat the layup preferably using a plurality of high frequency heating units in series. The patent described heating the mat adjacent to its middle to between 50 and 70.degree. C. or higher. The so preheated layup is subsequently pressed in a further press (unheated) and then a finishing press completes the consolidation by pressing under elevated temperature and pressure conditions.
U.S. Pat. No. 4,293,509 issued Oct. 6, 1981 to Bucking is similar to the above-described Lamberts et al patent in that it applies high frequency preheating. The concept of this patent is to apply high frequency preheating to heat the layup from the inside and cause the flow of steam out of the layup. The layup after the preheating stage maybe prepressed then it is finally pressed at elevated temperature by a steam press wherein the steam from the press penetrates the layup. Again, in all cases, the heating is as uniform as possible over the full width of the layup.
U.S. Pat. No. 5,063,010 issued Nov. 5, 1991 to Fisher et al. describes yet another system of forming a fiber or chipboard and conditions the mat by applying steam in a preheating section to raise the temperature and moisture content of the mat and the so conditioned mat is subsequently heated and compressed to the consolidated end product (fiber or chipboard).
The use of microwave energy to heat a layup is disclosed in U.S. Pat. No. 4,018,642 issued Apr. 19, 1997 to Pike et al. In this system, microwave energy is used to cure the resin in a very short period of time compared to conventional hot presses while avoiding the arcing and tracking problems that are involved with radio frequency heating. The microwave energy is applied either at the time of pressing or slightly before.
U.S. Pat. No. 4,456,498 issued Jun. 26, 1984 to Churchland describes a microwave heating system where the microwaves are introduced between a pair of pressure applying belts and travel across the width of the layup being compressed by the belts to heat the layup and cure the resin. This system has been commercially employed to manufacture a product sold by Trus Joist Macfillan, a Limited Partnership, under the trademark Parallan.RTM.. This patent discloses the particular type of pressing system for which the present invention is a significant improvement.
U.S. Pat. No. 5,892,208 issued Apr. 6, 1999 to Harris et al. discloses a system for heating a layup using circular magnetic mode microwave energy to uniformly heat the layup across substantially its full width.
The application of microwave energy between press belt as shown for example, in U.S. Pat. No. 4,456,498 of Churchland, applies the microwave energy from a pair of opposite side faces of the layup being pressed while the layup is under pressure, thus the microwave power is applied through supporting windows and penetrates into the layup from opposite sides. These systems generally heat the layup to a greater degree adjacent to the windows than the axial center of the layup so that if care is not taken uniformity of the product may be affected. Uniformity is also sometimes affected by local over heating of the layup to generate steam in pockets that tend to blow, disrupting the surface of the consolidated end product.